As heat-insulating core materials for vacuum insulation materials, core materials each of which is composed of one or more inorganic fiber mats of a material having a high heat-insulating effect, such as glass wool or rock wool, have been widely adopted conventionally. JP-A-60-14695 (published: Jan. 25, 1985) and JP-A-2001-108186 (published: Apr. 20, 2001) disclose that an organic binder is applied to an inorganic fiber mat upon production of such a core material so that the filling of the resulting core material into an envelope would be facilitated upon production of a vacuum insulation material.
According to the processes disclosed in the above patent publications, a centrifugal process or flame attenuation process is generally used to form a molten inorganic material into fibers such that the fibers are caused to accumulate or are collected to provide a mat-shaped preform. After a binder is sprayed onto the mat-shaped preform, the mat-shaped preform is formed under pressure and heat into a core material for a vacuum insulation material.
In a mat-shaped preform obtained by the above-described centrifugal process or the like, however, fibers are oriented in a three-dimensionally entangled form because they are drawn as relatively long fibers and are caused to accumulate in bent forms. When the mat-shaped preform with a binder applied thereon is formed under pressure and heat into a core material, the core material is inserted into an envelope and the resulting vacuum insulation material is filled in a heat-insulating casing, the vacuum insulation material is accompanied by a problem that its heat insulating properties are poor because in the core material, the fibers are fixedly bonded together in the above-described form and are not oriented perpendicularly relative to a direction in which heat is conducted.
Moreover, the vacuum insulation material obtained by the above-described process involves another problem that its surfaces are not smooth. When the vacuum insulation material is filled in a heat-insulating casing, spacings are formed between the inner walls of the casing and the vacuum insulation material due to rugged surfaces of the vacuum insulation material, thereby developing a further problem that the vacuum insulation material is inferior in heat insulating properties. In JP-A-9-4785 (published: Jan. 7, 1997), on the other hand, a process is disclosed to have fibers, which make up a core material, oriented perpendicularly relative to a direction in which heat is conducted. According to this process, fibers of 1 mm or shorter lengths are dispersed in water and are then collected together into a mat-shaped form. This process is, however, accompanied by a problem that it requires time and labor.